U.S. patent application number 10/543567 was filed with the patent office on 2006-05-11 for reconstruction of local patient doses in computed tomography.
This patent application is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Henrik Botterweck, Lothar Spies.
Application Number | 20060098856 10/543567 |
Document ID | / |
Family ID | 32798995 |
Filed Date | 2006-05-11 |
United States Patent
Application |
20060098856 |
Kind Code |
A1 |
Botterweck; Henrik ; et
al. |
May 11, 2006 |
Reconstruction of local patient doses in computed tomography
Abstract
In order to reduce an x-ray dose applied to a patient, it is
necessary to know the dose absorbed by the patient. According to
the present invention, there is provided a method of determining a
local patient dose applied to a patient where after the
reconstruction of the scan data into a diagnostic image, the scan
data are backprojected into the patient volume, using the
attenuation information of the diagnostic image to form a spatially
varying photon fluence map. In parallel, the diagnostic image is
segmented into anatomical structures to which dose-weighting
factors are assigned. The locally absorbed dose is then calculated
on the basis of the fluence map and the corresponding dose
weights.
Inventors: |
Botterweck; Henrik; (Aachen,
DE) ; Spies; Lothar; (Aachen, DE) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
595 MINER ROAD
CLEVELAND
OH
44143
US
|
Assignee: |
Koninklijke Philips Electronics
N.V.
Groenewoudsweg 1
Eindhoven
NL
BA-5621
|
Family ID: |
32798995 |
Appl. No.: |
10/543567 |
Filed: |
January 19, 2004 |
PCT Filed: |
January 19, 2004 |
PCT NO: |
PCT/IB04/00176 |
371 Date: |
July 27, 2005 |
Current U.S.
Class: |
382/131 |
Current CPC
Class: |
A61B 6/032 20130101;
Y10S 128/922 20130101; A61N 5/1031 20130101 |
Class at
Publication: |
382/131 |
International
Class: |
G06K 9/00 20060101
G06K009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2003 |
EP |
03100190.2 |
Claims
1. A method of determining a local patient dose applied to a
patient in computed tomography, the method comprising the steps of:
segmenting a diagnostic image of an area of interest of the
patient; and determining a dose image showing local patient doses
applied to the patient by using the segmented diagnostic image.
2. The method according to claim 1, wherein the step of segmenting
the diagnostic image of the area of interest of the patient further
comprises the steps of: segmenting the diagnostic image into
anatomical structures in the area of interest of the patient;
assigning dose-weighting factors to the anatomical structures
segmented in the diagnostic image.
3. The method according to claim 2, further comprising the step of:
determining a fluence map on the basis of the diagnostic image of
an area of interest of the patient.
4. The method according to claim 3, wherein the step of determining
the dose image showing local patient doses applied to the patient
further comprises the step of: determining the dose image showing
local patient doses applied to the patient on the basis of the
fluence map and the dose-weighting factors.
5. The method according to claim 3, wherein the diagnostic image
has grey levels indicating Hounsfield units and is determined from
scan data obtained from scanning the area of interest of the
patient and the fluence map is determined by filtering the scan
data and back-projecting it using information obtained from the
previously determined diagnostic image.
6. An image processing device, comprising: a memory for storing
scan data; an image processor for determining a local patient dose
applied to a patient in computed topography, which processor
performs the following operation: segmenting a diagnostic image of
an area of interest of the patient; and determining a dose image
showing local patient doses applied to the patient by using the
segmented diagnostic image.
7. The image processing device according to claim 6, further
performing the operation of: segmenting the diagnostic image into
anatomical structures in the area of interest of the patient;
assigning dose-weighting factors to the anatomical structures
segmented in the diagnostic image.
8. The image processing device according to claim 7, further
performing the operation of: determining a fluence map on the basis
of the diagnostic image of an area of interest of the patient; and
determining the dose image showing local patient doses applied to
the patient on the basis of the fluence map and the dose-weighting
factors.
9. The image processing device according to claim 6, wherein the
image processing device is part of a computed tomograpy system.
10. Computer program for an image processing device, for
determining a local patient dose applied to a patient in computed
tomography, the computer program comprising the steps of:
segmenting a diagnostic image of an area of interest of the
patient; and determining a dose image showing local patient doses
applied to the patient by using the segmented diagnostic image.
Description
[0001] The present invention relates generally to methods and
apparatus for "computed tomography" (CT) and other radiation
imaging systems and more particularly to a method of determining a
local patient dose applied to a patient in computed tomography, an
imaging processing device and a computer program for an image
processing device.
[0002] In at least some CT imaging system configurations, an x-ray
source projects a fan-shaped beam such that it is collimated to lie
within an X-Y plane of a Cartesian coordinate system, which is
usually referred to as the "imaging plane". The x-ray beam passes
through an object being imaged, such as a medical patient. The
beam, after being attenuated by the object, impinges upon an array
of radiation detectors. The intensity of the attenuated beam
radiation received at a detector array is dependent upon the
attenuation of the x-ray beam by the object being imaged. Each
detector element of the detector array generates an electrical
signal indicating the beam attenuation at the detector location.
The attenuation measurements from all the detectors are acquired
separately to produce a transmission profile and represent scan
data.
[0003] In known third generation CT systems, the x-ray source and
the detector array are rotated with a gantry within the imaging
plane and around the object to be imaged, such that an angle at
which the x-ray beam intersects the object changes constantly. The
used x-ray sources usually include x-ray tubes, which emit the
x-ray beam at a focal spot. X-ray detectors may include a
collimator for collimating x-ray beams received at the detector, a
scintillator adjacent to the collimator and photodetectors adjacent
to the scintillator. A group of x-ray attenuation measurements,
i.e. projection data from the detector array at one ganrty angle is
referred to as a "view". A "scan" of the object for generating the
necessary scan data comprises a set of views made at different
gantry angles, or view angles, during one revolution of the x-ray
source and detector.
[0004] In an axial scan, the projection data is processed to
construct an image that corresponds to a two-dimensional slice
taken through the object. A known method for reconstructing an
image from a set of projection data or scan data is the filtered
back projection technique. This process converts the attenuation
measurements from a scan into integers called "CT numbers" or
"Hounsfield units", which are used to control grey values of a
corresponding diagnostic image.
[0005] In other words, in the known CT scanners, the recorded
projections acquired during an object or patient scan (scan data)
are mathematically reconstructed into a tomographic image with grey
levels indicating Hounsfield units. The tomographic image is used
by, for example, a clinician for diagnostic purposes and is
referred to as the diagnostic image in the following.
[0006] Currently, attempts are made to reduce doses applied to
patients during CT scans. However, an identification of an
exceeding dose and an optimation of the dose applied to the patient
during the scan requires the exact knowledge of the doses actually
applied to the patient during the scan.
[0007] The program "WinDose" (Kalendar, W. A., Schmidt B., Zankl
M., Schmidt M. (1999), A PC program for estimating organdose and
effective dose values in computed tomography. European Radiology 9:
555-562) is a PC program for estimating organdose and effective
dose values in computed tomography and calculates the patient dose
applied to the patient, on the basis of only a few parameters of
the patient. Primarily, these parameters are referring to the
circumference and weight of the patient and the settings of the
x-ray tube, such as the applied power (mA, kVp). Then, on the basis
of tables, and by using a Monte Carlo simulation, which has been
computed with respect to a standard patient having a standard
shape, an integral dose of the most important organs may be
determined. However, this computer program does not deliver
sufficient results in case the patient differs from the standard
data, for example in pediatrics.
[0008] WO 00/07667 relates to a radiotherapy verification system,
wherein a dose delivered to the patient may be computed on the
basis of a model of the patient to estimate values of energy
fluence prior to absorption by the patient and overlapping of the
various radiation beams passing through the patient. A model may be
constructed from a known geometry of the radiation therapy machine
and estimated properties of the patient or standard patient as
deduced from a pre-treatment tomography. Accordingly, in order to
compute a dose delivered to the person in the system known from WO
00/07667, either a standard patient has to be used for the
computation of the dose resulting in the same insufficient results
as the program "WinDose" or a pre-treatment tomogram has to be
carried out which increases the overall dose applied to the
patient.
It is an object of the present invention to determine and minimize
a local patient dose applied to a patient in computed
tomography.
[0009] According to an exemplary embodiment of the present
invention, this object is solved with a method of determining a
local patient dose applied to a patient in computed tomography
comprising the steps of segmenting a diagnostic image of an area of
interest of the patient and determining a dose image showing local
patient doses applied to the patient by using the segmented
diagnostic image. Due to the segmentation of the diagnostic image,
a local dose delivery to critical and dose-sensitive organs may be
determined very accurately and may enable an improved patient dose
management.
[0010] According to another exemplary embodiment of the present
invention as set forth in claim 2, anatomical structures are
segmented in the diagnostic image and dose-weighting factors are
assigned to the anatomical structures. Advantageously, this allows,
for example, for distinguishing between sensitive and non-sensitive
organs of the patient and allows for a very exact determination of
the local patient dose.
[0011] According to yet another exemplary embodiment according to
claim 3 of the present invention, a fluence map is determined.
Advantageously, this allows a clinician to immediately determine
and to monitor a local dose delivery to critical and dose-sensitive
organs of the patient.
[0012] According to another exemplary embodiment of the present
invention according to claim 4, the dose image is determined on the
basis of the fluence map and the dose-weighting factors. This
allows for a very accurate determination of the local patient dose
applied to the patient while being very effective and efficient
with respect to computational efforts. Thus, this exemplary
embodiment of the present invention provides for a very simple and
fast method to determine the local patient dose.
[0013] According to another exemplary embodiment of the present
invention as set forth in claim 5, the diagnostic image has grey
levels indicating Hounsfield units and the fluence map is
determined by filtering the scan data appropriately and
back-projecting it using the diagnostic image. This allows for a
simple determination of the fluence map. Further, this exemplary
embodiment of the present invention allows for a very accurate
determination of the fluence map.
[0014] According to another exemplary embodiment of the present
invention, an image processing device is provided with the features
of claim 6, which allows for a very accurate determination of local
patient dose.
[0015] Further exemplary embodiments of the image processing device
according to the present invention, as set forth in claims 6, 7 and
8, provide for a fast and efficient determination of the local
patient dose applied to the patient while minimizing computation
efforts and while being very accurate.
[0016] According to another exemplary embodiment of the present
invention as set forth in claim 10, a computer program for an image
processing device is provided, executing the method according to
the present invention.
[0017] As described above, CT scanner devices usually produce a
diagnostic image on the basis of the recorded projections acquired
during a patient scan, with grey levels indicating Hounsfield
units. However, Hounsfield units are proportional to linear
attenuation coefficients of the underlining anatomical structure,
but differ from the energy absorbed locally in the patient.
Consequently, the absorbed dose distribution will deviate in a
non-linear way from the underlying diagnostic image. According to
the present invention, a method and apparatus are provided which
are able to reconstruct the local dose distribution on the basis of
the registered projections (scan data). It may be seen as the gist
of the invention that the method/apparatus makes twofold use of the
scan data as follows: After reconstruction of the scan data into a
diagnostic image, for another time, the scan data are
back-projected into the patient volume using the attenuation
information of the diagnostic image forming a spatially varying
photon fluence map as being produced by the x-ray beam incident on
the patient. In parallel, the diagnostic image is segmented into
anatomical structures (regions with approximately constant
attenuation). Dose-weighting factors are assigned which account for
the difference between locally absorbed energy and photon
attenuation. The locally absorbed dose is then calculated on the
basis of the fluence map and the corresponding dose weights.
[0018] These and other aspects of the present invention will become
apparent from and elucidated with reference to the embodiments
described hereinafter.
[0019] Exemplary embodiments of the present invention will be
described in the following, with reference to the following
drawings:
[0020] FIG. 1 shows a pictorial view of a CT imaging system.
[0021] FIG. 2 shows a block schematic diagram of the system
illustrated in FIG. 1.
[0022] FIG. 3 is a flow chart illustrating an exemplary embodiment
of a sequence of steps executed by the CT imaging system of FIGS. 1
and 2, to determine a local patient dose applied to a patient.
[0023] FIG. 4 is a flow chart illustrating another exemplary
embodiment of a sequence of steps executed by the CT imaging system
of FIGS. 1 and 2, to determine a local patient dose applied to a
patient.
[0024] Referring to FIGS. 1 and 2, a computed tomography (CT)
imaging system 1 is shown. It includes a gantry 2, representative
of a "third generation" CT scanner. Gantry 2 has an x-ray source 4
that projects a beam of x-rays 6 towards a detector array 8 on the
opposite side of the gantry 2. The detector array 8 is formed by a
plurality of detector elements 10, which together sense the
projected x-rays that pass through an object, such as depicted in
FIG. 1, a medical patient 12. Each detector element 10 produces an
electrical signal that represents an intensity of an impinging
x-ray beam and hence the attenuation of the beam as it passes
through the object or patient 12. During a scan to acquire x-ray
projection data, the gantry 2 and the components mounted thereon,
namely the x-ray source 4 and the detector 8 with the detector
elements 10, rotate about a center of rotation 14. In the
embodiment shown in FIG. 2, a plurality of detector elements 10 are
arranged in one row, so that projection data corresponding to a
single image slice is acquired during a scan. According to another
embodiment, the detector elements 10 may be arranged in a plurality
of parallel rows, so that projection data corresponding to a
plurality of parallel slices can be acquired simultaneously during
a scan.
[0025] The rotation of the gantry 2 and the operation of x-ray
source 4 are controlled by a control mechanism 16 of the CT system
1. The control mechanism 16 includes an x-ray controlling unit 18,
which provides the necessary power and timing signals to the x-ray
source 4 and a gantry controlling unit 20, which controls the
rotational speed and position of gantry 2 by generating and
providing respective control signals to the drive of the gantry
2.
[0026] Reference number 22 designates a data acquisition system
provided in the control mechanism (16). The data acquisition system
22 samples analogue data from the detector elements 10 and converts
the data to digital signals for subsequent processing.
[0027] Furthermore, there is provided an image reconstructuor 24
which receives samples in digitised x-ray data from the data
acquisition system 22 and performs a high-speed image recognition.
Also, there is provided a dose reconstructor 40 for reconstructing
a dose which is connected to the data acquisitions system 22 and to
the image reconstructor 24. Instead of providing a separate image
reconstructor 24 and a separate dose reconstructor 40, the function
and operation of the image reconstructor 24 and the dose
reconstructor 40 may be combined in a separate device or may also
be accommodated in the computer 26.
[0028] The reconstructed image is applied as an input to the
computer 26 which stores the image in a mass storage device 28,
such as a hard disc drive or a floppy drive.
[0029] The computer 26 also receives commands and scanning
parameters from an operator via an input device 30, such as a
keyboard or pointer device. Furthermore, there may be provided an
output device 32 such as a display or printer, in order to allow
the operator to observe the reconstructed image and other data
provided by the computer 26. The operator supplied commands and
parameters are used by computer 26 to provide control signals and
information to the data acquisition system 22, to the x-ray
controller 18 and to the gantry controlling unit 20. Furthermore,
the computer 26 generates output signals output to a table drive
controller 34 for controlling a movement, position and inclination
of the table 36, by providing respective control signals to drive
units of the table 26. Thus, the computer 26 controls the position
and movement of the patient 12 in a gantry opening 38 of the gantry
2. In particular, by the movement of the table 36, the patient 12
is moved through the gantry opening 38.
[0030] The data acquisition system 22, the gantry controlling unit
20, the x-ray controlling unit 18, the image reconstructor 24, dose
reconstructor 40 and the table drive controller 34 may all be
realized by means of suitable processors or programmable logic
controllers, such as EPLDs distributed by ALTERA in combination
with respective power amplifiers. However, all the operations and
functions provided by the data acquisition system 22, the gantry
controlling unit 20, the x-ray controlling unit 18, the image
reconstructor 24, the dose reconstructor 40, the output device 32,
the data storage 28, the input device 30, the computer 26 and the
table drive controller 34, may be realized by means of a computer
such as a personal computer with, for example, a Pentium
processor.
[0031] Image information or data provided by the data acquisition
system 22 and/or the image reconstructor 24, is referred to as
"scan data" in the following:
[0032] FIG. 3 shows a flow chart illustrating an exemplary
embodiment of a sequence of steps of a method according to an
exemplary embodiment of the present invention, executed by the CT
system shown in FIGS. 1 and 2, to determine a local patient dose
applied to the patient 12. The steps S1 to S11 are preferably
executed in the data acquisition system 22, the dose reconstructor
40, the image reconstructor 24 and/or the computer 26. For storing
and outputting, the output device 32 and the data storage 28 may be
used. After the start in step S1, the method continues to step S2,
where the scan data is obtained in known manner. Then, the method
continues to step S3, where the scan data is filtered in known
manner. Then, in a subsequent step S4, a tomographic reconstruction
on the basis of the filtered scan data is carried out, in order to
determine the diagnostic image of the area of interest of the
patient. Usually, the tomographic reconstruction is such that a
diagnostic image is generated with grey levels indicating
Hounsfield units. After generation of the diagnostic image, the
method continues to step S5, where the diagnostic image is output
to a user or clinician for, e.g. diagnostic purposes.
[0033] Then, the method continues to steps S6 to S9 where the data
is prepared for dose reconstruction.
[0034] In step S6, the diagnostic image is segmented into
anatomical structures assigned to its voxels. In other words, the
diagnostic image is segmented to "material", where a raw
classification (e.g. metal, bone, water, air) based on
Hounsfield-thresholds may be sufficient for this purpose.
[0035] In order to transform a fluence F to dose D, dose weighting
factors (as indicated as factor prior to the fluence exponential
function in the following formulae) are assigned to the anatomical
structures in step S7, based on the already obtained raw
segmentation according to material. This may be done by referring
to predetermined tables linking the respective anatomical
structures to predetermined dose-weighting factors. Anatomical
structures are, e.g. bones, muscles, spaces or accumulations of
blood.
[0036] Thus, for example a different dose-weighting factor may be
assigned to muscles which are less prone to be damaged by the x-ray
beam than, for examples organs like the liver or parts of the
brain. In general, the dose-weighting factors should be determined
such that they account both for differences between locally
absorbed energy and photon attenuation of the respective anatomical
structure and for biologically different sensitivity to dose
absorption.
[0037] In step S8, the filtered scan data determined in step S3 is
back-projected in a way appropriatlely for dose reconstruction and
a fluence map is reconstructed on the basis of this
back-projection. Since the scan data is filtered in the same way as
in step S3 (for simplicity, but not necessarily with the same
filter kernel) as for reconstructing the Hounsfield image, the
filtered data from image reconstruction may be saved in step S3 and
re-used here. Advantageously, this allows to reduce the amount of
arithmetic calculations.
[0038] A straight-forward calculation of total absorbed dose D at a
position x in the patient volume carried out in step S8
(S6-S9)--directly expressed by means of the original scan data--can
be described with the following equations: D .function. ( x _ ) =
.times. S 0 .times. .mu. a * .intg. d .theta. .times. .times. e -
.intg. 0 .infin. .times. .times. d .lamda. .times. .times. .mu.
.function. ( x _ + .lamda. .times. e _ .theta. ) .times. ( 1 ) = S
0 .times. .mu. a * .intg. d .theta.e - .intg. 0 .infin. .times.
.times. d .lamda. .times. .intg. d t .function. [ .sigma.
.function. ( t , ) * k .function. ( ) ] .times. ( ( x _ + .lamda.
.times. e _ .theta. ) .times. e _ .times. 1 t ) .times. .times. ( 2
) ##EQU1##
[0039] .mu..sup..alpha.: material dependent absorption or scatter
kernel (3)
[0040] .theta.: gantry angle, i.e x-ray source position (4)
[0041] e.sub..theta.:unit vector in direction .theta. (5)
[0042] e.sub.t.sup..perp.: unit vector orthogonal to direction t
(6)
[0043] .sigma.( ): scan data (7)
[0044] k( ): filter kernel (8) d .function. ( x _ , .theta. ) =
.intg. d t .times. .intg. 0 .infin. .times. .times. d .lamda.
.times. [ .sigma. .times. ( t , ) * k .function. ( ) ] .times. ( xe
_ .times. 1 t + .lamda. .times. .times. sin .function. ( .theta. -
t ) ) .ident. g .function. ( t , .lamda. _ , x .times. e _ .times.
1 t ) ( 9 ) = .intg. 0 .infin. .times. .times. d .lamda. ~ .times.
.intg. d t .times. 1 sin .function. ( .theta. - t ) .times. g
.function. ( t , .lamda. ~ , x .times. e _ .times. 1 t ) .times. (
10 ) = .intg. d .lamda. ~ .times. h .function. ( .theta. , .lamda.
~ , x _ ) .times. ( 11 ) ##EQU2##
[0045] g( ): convolution as known from image reconstruction
(12)
[0046] h( ): point-wise convolution with inverse sinus (13)
[0047] In this realisation example, instead of ray-tracing as will
be described with reference to FIG. 4, the filtered scan data is
re-used for dose reconstruction. The fluence F in x is integrated
over all views similar to the back-projection method of image
reconstruction.
[0048] Then, the method continues to step S9, in which a dose image
is determined in accordance with the above formulae. The dose image
determined in step S9 shows local patient doses applied to the
patient. The dose image is determined by using the dose-weighting
factors and the fluence map.
[0049] After the determination of the fluence map, the method
continues to step S10, where the dose-image is output. Then, the
method continues to step S11, where it ends.
[0050] According to another exemplary embodiment of the method of
determining a local patient dose applied to a patient according to
the present invention, the reconstruction of the fluence map and
the segmentation of the diagnostic image into anatomical structures
may be carried out in parallel. This allows for a very fast
computation of the dose image.
[0051] FIG. 4 shows a flow chart illustrating another exemplary
embodiment of a sequence of steps of a method according to an
exemplary embodiment of the present invention, executed by the CT
system shown in FIGS. 1 and 2, to determine a local patient dose
applied to the patient 12. Step 12 is preferably executed in the
data acquisition system 22, the dose reconstructor 40, the image
reconstructor 24 and/or the computer 26. For storing and
outputting, the output device 32 and the data storage 28 may be
used. Steps S1 to S7 and S10 to S11 are the same as described with
reference to FIG. 4.
[0052] Firstly, in steps S6 and S7, the reconstructed image is
segmented according to "material", where a raw classification (e.g.
metal, bone, water, air) based on Hounsfield-thresholds is
sufficient for this purpose, but more elaborated methods may be
used, and dose-weighting factors are assigned to the anatomical
structures. Then, in step S12, for each position of the gantry
where a view has been generated, the x-ray source spectrum as known
from calibration data is ray-traced through the simulated patient
volume as known from step S4, separated into voxels with assigned
material and corresponding attenuation coefficients. The absorbed
dose is integrated for each voxel. Then, a dose image is determined
which is output in the subsequent step S10. In other words, in step
S12 the x-ray spectrum is forward-projected and the fluence map is
reconstructed on basis of the segmented diagnostic image and then
the dose image is determined that shows local patient doses applied
to the patient using the dose-weighting factors and the fluence
map.
[0053] In brief, according to the present invention, the
method/apparatus make two-fold use out of the measured scan data.
Namely, the scan data is reconstructed into a diagnostic image.
Also, the scan data is back-projected into the patient volume,
using the attenuation information of the diagnostic image forming a
spatially varying photon fluence map, as being produced by the
x-ray beam incident on the patient. In parallel, the diagnostic
image may be segmented into anatomical factors, to which
dose-weight factors are assigned which account for the difference
between locally absorbed energy and photon attenuation. The locally
absorbed dose is then calculated on the basis of the fluence map
and the corresponding dose weights.
[0054] Preferably, the above method/apparatus may be used for CT
systems.
* * * * *